1. Field of the Invention
The invention relates to a device for the plastic sheathing of connections, which are produced by circular weld seams, of a pipeline comprising steel pipes which are provided with a factory-fitted plastic insulation which is axially spaced from the weld seam to form an axial region between the insulation of respective pipes, the device including a split injection mold which can be connected to an injection-molding fixture so that it covers the axial region in the manner of a sleeve and extends over the plastic insulation on the pipes.
2. Description of the Related Art
WO 90/02038 has disclosed a device for reinsulating steel pipes which have in each case been connected to one another by a circular weld seam and have already been provided with a factory-fitted plastic insulation, for example comprising polyethylene or polypropylene. To ensure that the factory-fitted plastic insulation is not damaged by the welding operation, the pipes have in each case been left without a coating over a short section of their axial length at the ends. After two pipes have been connected by a circular weld seam, the uncoated partial lengths have to be reinsulated in the region of the connection in order to prevent external corrosion. For this purpose, an injection mold is provided, which is fitted around the steel pipes in the manner of a sleeve in the region of the circular weld seam and can be connected to an injection-molding fixture for an injection-moldable plastic via a gate device. Its axial length is dimensioned in such a way that it covers the region of the partial length which is to be reinsulated with an overlap and its two ends rest on the existing factory-fitted plastic sheathing of the two connected steel pipes. To allow rapid fitting of the injection mold onto the connecting region of the pipeline and to make it easy to remove the mold again from the pipeline after reinsulation has been completed, the mold is of split design. Typically, it comprises two half-shells, which are held together, for example, by a hinge, are placed around the pipeline and are then connected to one another at the open part by a locking means, so that it is secured on the section of pipe. The injection mold is designed substantially in the form of a hollow cylinder, with its internal diameter, in the region of the weld seam region which is to be reinsulated, corresponding to the external diameter of the plastic sheathing which is to be produced. It therefore surrounds the reinsulation region at a certain distance therefrom. By contrast, at its ends the internal diameter of the injection mold is smaller and corresponds to the external diameter of the factory-fitted plastic insulation.
In practical operation, however, there is often a certain annular gap between the factory-fitted plastic insulation and the inner surface of the injection mold at the ends of the latter. This annular gap causes problems, since some of the plastic which is injected into the injection mold escapes to the outside through this gap, the size of which should ideally be zero. This leads to a pressure loss in the injection mold, which is disadvantageous for correct realization of the reinsulation and may lead to quality losses (e.g. inclusion of air bubbles in the coating). The larger the diameter of the steel pipes and the greater the deviations from the ideal pipe geometry (e.g. ovality), the more serious this problem becomes.
It is known from U.S. Pat. No. 3,387,075 A that, in a process for producing domestic sieves from plastic by injection molding, the flow front of the plastic material undesirably comes to a standstill as a result of cooling. Furthermore, DE 34 40 326.4 has disclosed an injection-molding process for the production of domestic sieves in which a sieve mesh, which is curved substantially in the shape of a hemisphere and is made from wire, is clamped in a two-part injection mold in the region of an encircling collar on a narrow annular surface before the collar is extrusion-coated with plastic and a handle is injection-molded on. Cooling of the mold or of the clamped-in sieve mesh reliably prevents plastic from undesirably escaping beyond the region of the clamped-in zone. These documents do not provide any indications whatsoever with regard to reinsulation of steel pipes in the region of a circular weld seam.